(1) Field of the Invention
The present invention relates to an obliquely oriented polypropylene cross film. More particularly, the present invention relates to an obliquely oriented polypropylene cross film comprising a plurality of polypropylene films formed of a polypropylene/polyethylene composition comprising polypropylene as the main component and having a molecular orientation in a certain oblique direction, which are laminated so that the molecular orientation directions of the films cross one another, and a fastening tape for a paper diaper, having excellent mechanical strength, heat resistance and softness, in which the above-mentioned cross film is used at least as a part of a base of the tape.
(2) Description of the Related Art
It is known that an obliquely oriented cross film formed by laminating two polyethylene films having a molecular orientation in a certain oblique direction so that the molecular orientation direction cross each other is used for various packaging materials. For example, Japanese Utility Model Publication No. 19087/81 and Japanese Utility Model Publication No. 20590/86 teach that a cross film as mentioned above is used as a packaging bag for a photosensitive material, and Japanese Utility Model Publication No. 13637/85 teaches that a cross film as mentioned above is used as a self-supporting packaging bag.
High-density polyethylene is used as the starting material for these known obliquely orientated cross films. The reason is that since high-density polyethylene has a sufficient melt tension at the melt extrusion, the film-forming property is excellent. Since this obliquely oriented cross film composed of high-density polyethylene has a resistance against impact and rupture and a good balance of tensile strength and elongation, the cross film is used for various packaging materials. However, the cross film is defective in that the heat resistance is poor, and therefore, the use is limited to packaging materials as mentioned above.
It is well known that polypropylene is superior to polyethylene in the heat resistance, but polypropylene involves a serious technical problem. Namely, if it is intended to shape polypropylene into an obliquely oriented film, since the melt tension is insufficient at the extrusion of polypropylene from an extruder, it is impossible to obtain a self-supporting, obliquely oriented film.
An adhesive tape fastener used as the fastening member of a paper diaper comprises a fastening tape for fastening the front and back parts of the paper diaper to maintain a trunk-like shape, a release tape temporarily fixed to the paper diaper proper before the use and acting as the temporarily fixing base when released and used, and an intertape located between the fastening tape and the release tape before the use, which is bonded to the front part of the paper diaper during the use to act as the temporarily fixing member for the fastening tape and can release the fastening tape repeatedly.
As the base of the fastening tape, there have been used a nonwoven fabric, a paper, a polyethylene film, a polypropylene film and the like.
For the fastening tape, it is required that the tensile strength and tear strength of the base should be high for preventing breaking of the tape, the interface between the sticking agent and the base should be stabilized for preventing peeling of the sticking material, and the softness and heat resistance should be high.
However, the above-mentioned conventional bases are poor in the tensile strength or tear strength and are also defective in that the softness and heat resistance are poor. If the tensile strength or tear strength is poor, while binding and peeling are repeated between the fastening tape and the intertape during the use of the paper diaper, the fastening tape is broken. If the film is poor in the softness, an underwear of a baby or the hand of a handler is often damaged by the end or section of the film during the use of the diaper. Furthermore, if the heat resistance of the film is poor, the film is curled by heating at the step of coating the sticking agent on the molten surface of the film, and the operation of coating the sticking agent becomes difficult.